In most cases, monocrystalline silicon essential for the manufacture of semiconductor devices and the like is grown by the FZ method or the CZ method using as a raw material a polycrystalline silicon rod produced by the Siemens method or polycrystalline silicon masses obtained by crushing the polycrystalline silicon rod. The Siemens method is a method involving bringing a silane raw material gas such as trichlorosilane or monosilane into contact with a heated silicon core wire to vapor-phase-grow (deposit) polycrystalline silicon on the surface of the silicon core wire by a CVD (Chemical Vapor Deposition) method.
For polycrystalline silicon masses, a polycrystalline silicon rod synthesized by the Siemens method is removed from a reactor and then fractured, and the fractured material is subjected to chemical solution etching with fluoronitric acid or the like in order to remove contaminants adhering to the surface of this fractured material to achieve a surface state with high cleanliness, and commercialized.
When a silicon material such as polycrystalline silicon masses is etched, generally, a mixed acid solution of hydrofluoric acid (HF) and nitric acid (HNO3) or a mixed acid solution comprising hydrogen peroxide (H2O2) in the above mixed acid solution is used, and each acid concentration in these mixed acid liquids is selected according to the concentrations and degrees of difficulty of removal of contaminants adhering to the silicon material to be washed, and the like.
The components of the mixed acid solution remain on the surface of the silicon material after such etching, and it is necessary to remove these residues by water washing and rinsing. But, when the concentrations of the residual components are high, it is difficult to completely remove the residual components by water washing and rinsing. When monocrystalline silicon is grown by the CZ method using as a raw material polycrystalline silicon masses on the surfaces of which residues are present, a problem is that a phenomenon such as crystal lines, signs of single crystallization, disappearing and being disarranged occurs, and the completeness of the crystal cannot be guaranteed.
The cause of this is not necessarily clarified, but this may be due to the nitrogen (N) component and fluorine (F) in the above mixed acid solution remaining on the surfaces of the polycrystalline silicon masses.
Particularly, an infinite number of cracks and crevices are present in polycrystalline silicon masses, and therefore when the mixed acid solution components enter these gaps, it is next to impossible to completely remove the mixed acid solution components. Such residual components are mainly three types, a nitrate ion component, a nitrite ion component, and a hydrofluoric acid ion component. According to measurement by the present inventors, the following result is also obtained. Even if polycrystalline silicon masses are immersed in pure water at about 40° C. and subjected to ultrasonic washing for 24 hours, these residual components cannot be completely removed.